1. Field of the Invention
The present invention relates to a backside-illuminated imaging device that illuminates light from the back side of a semiconductor substrate and reads electric charges, which are generated in the semiconductor substrate based on the light, from the front side of the semiconductor substrates, to perform imaging.
2. Description of Related Art
There has been proposed a backside-illuminated imaging device that illuminates light from the back side of a semiconductor substrate, accumulating electric charges, which are generated in the semiconductor substrate based on the light, in charge accumulating areas on the surface of the semiconductor substrate, and outputting a signal in accordance with the accumulated electric charges to the outside by a CCD circuit or a COMS circuit on the surface of the semiconductor substrate, to perform imaging.
The semiconductor substrate (photoelectric conversion area) of the backside-illuminated imaging device is required to have a thickness of about 10 μm in order to absorb most of the visible light. For that reason, when the backside-illuminated imaging device is manufactured, a manufacture sequence is as follows. That is, elements of the charge accumulating areas, CCD, and the like are first formed on the front side of the semiconductor substrate with some thicknesses; a support substrate is adhered to the elements with an adhesive; and then the semiconductor substrate is etched from the back side thereof until the thickness of the semiconductor substrate becomes about 10 μm. After the etching, elements of a color filter, a microlens, and the like in accordance with the elements formed on the front side of the semiconductor substrate is formed on the back side of the semiconductor substrate.
JP-A-2005-285988 discloses a technique for adhering a support substrate to a semiconductor substrate with an adhesive, and then the semiconductor substrate is etched from the back side of the semiconductor substrate.
In order to adhere a semiconductor substrate to a support substrate, an adhesive (epoxy resin) or the like made of an organic material is generally used. There is a technique for directly adhering a semiconductor substrate to a support substrate, but an apparatus for performing the adhering is expensive and the surface of the semiconductor substrate is required to be almost ideally flat. For that reason, troublesome problems arise in that a flattening process by a CMP is necessary, a dummy pattern for reducing a dishing phenomenon is introduced, a dummy pattern shape is optimized, or the like. Therefore, the adhesive made of an organic material is advantageous in terms of manufacturing cost.
In a case of the backside-illuminated imaging device, it is required to form a color filter, a microlens, and the like with high positional precision on the back side of the semiconductor substrate, with reference to a side on which the support substrate is not adhered to the semiconductor substrate. In order to achieve the high positional precision, an appearance of the support substrate attached to the semiconductor substrate is required to be flat to the extent that the support substrate is suspended on a photolithographic equipment such as a stepper. A problem does not occur since the flatness of the support substrate can be achieved as much as the flatness of the semiconductor substrate. However, in a case where the adhesive made of an organic material is interposed between the support substrate and the semiconductor substrate, there is a possibility that minute tilting (flatness error between wafers) occurs between the support substrate and the semiconductor substrate. In the case of the photolithographic equipment capable of making a minuter pattern, a margin is small in unevenness of a focal plane, and in the photolithographic equipment capable of achieving a minimum line width of about 0.2 μm, a photoresist shape formed by the photolithographic equipment may become a quite different shape when unevenness of about several μm occurs in the wafer surface. Accordingly, in order to form the front-side elements and the back-side elements with the high positional precision, the adhering of the support substrate and the semiconductor substrate has to be performed with high precision so as to allow the thickness of a space between the support substrate and the semiconductor substrate to be uniform. In the technique disclosed in JP-A-2005-285988, there is no guarantee that the thickness of the space between the support substrate and the semiconductor substrate is uniform. Accordingly, deterioration in characteristics or yields caused due to the positional mismatch between the front-side elements and the back-side elements may occur. Moreover, color mixture caused due to deformation of the color filter or the microlens shape may increase.